Zsombor Papp wrote: > > c.) In general, you can. In the case of Ethernet interfaces for > instance, > you can specify encapsulation on a subinterface level.
On an Ethernet subinterface you can specify the VLAN tagging method using an encapsulation command. That is specific to a router that is doing inter-VLAN routing and needs to tag frames and you want to specify ISL or IEEE 802.1Q tagging. In general you can't specify the encapsulation used on Ethernet interfaces or subinterfaces. 802.1Q isn't really an encapsulation, in that it inserts a shim rather than surrounding the original frame, but that's for another discussion. > In fact > I seem to > remember that you *had* to specify it on a subinterface level > for a while, > and it was in fact the encapsulation type that selected the > subinterface. > The reason for this behavior is that "Ethernet" has two > sub-layers within > Layer 2, so even there you have an implied encapsulation > assigned to the > physical interface (the lower of the two sub-layers, the IEEE > 802.3/Ethernet II format), and only the higher layer (layer > 802.2) > encapsulation is assigned to a subinterface. You can't tell an Ethernet interface to use 802.2 except within the IPX network commands. Maybe you're thinking of the 802.1Q VLAN tagging standard. > There was a > lengthy discussion > about Ethernet encapsulations on this list a few days ago, btw. But that's not what the discussion said! ;-) Ethernet encapsulation depends on the upper layer, which isn't like encapsulation on a serial interface. IP uses Ethernet Version 2: Dest Src Type (no sublayers) BPDUs use IEEE 802.3 and 802.2 Dest Src Length DSAP SSAP Control CDP, VTP, AppleTalk use IEEE 802.3, 802.2, SNAP Dest Src Length DSAP SSAP Control SNAP IPX can use any of those three or novell-ether (raw) Dest Src Length Encapsulation is set with the IPX network-layer commands becuase IPX supports 4 methods. The only major exception is for VLAN tagging. Then you can specify encapsulation with a subinterface to say whether you want ISL or 802.1Q. Ethernet encapsulation behavior isn't much like encapsulation on serial interfaces, which can only be one of the major categories: HDLC, PPP, Frame Relay. But Frame Relay has 2 varieties. Priscilla > > At this point, the more specific question is in order: > > d.) Why can I not specify any encapsulation on a Serial > sub-interface in IOS? > > Well, perhaps because the encapsulation you specified at the > physical level > (see above why you have to have that) took care of everything > that you > would characterize as "encapsulation". This of course doesn't > mean that all > the packets assigned to a subinterface are the same, but for > some reason we > don't speak about "IP encapsulation" vs. "IPX encapsulation" > and the like. > > >If it's possible to assign a network address ( IP / IPX viz., > ) to the > >FR sub-interface why not be able to specify encap as well..?? > > Network addresses don't specify the data format, they are the > data > themselves. If you wanted to ask "how come I can run IP and IPX > on the same > interface", then the answer is "because something at a lower > layer will > usually indicate what kind of packet you are receiving". > > Thanks, > > Zsombor > > At 08:55 AM 6/26/2003 +0000, Srivathsan Ananthachari wrote: > >If it's possible to assign a network address ( IP / IPX viz., > ) to the > >FR sub-interface why not be able to specify encap as well..?? > > > >I hope I'm not draggin it.../ > > > >Srivathsan > > > >-----Original Message----- > >From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On > Behalf Of > >Mwalie W > >Sent: Thursday, June 26, 2003 12:42 PM > >To: [EMAIL PROTECTED] > >Subject: RE: Subinterfaces [7:71421] > > > > > >Hi, > > > >Well, encapsulation is done prior to placing the > packets(Frames) to the > >physical medium - I guess it should be done on the physical > interface. > > > >I would be interested in what other members have to say, but I > think it > >makes sense that it should be on the physical interface. > > > >Mwalie > > Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=71456&t=71421 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
